PET (polyethylene terephthalate) is an industrially valuable polyester because of its excellent mechanical properties and relatively low cost. For example, PET has been widely used in various applications such as fibers, tire cords, bottles and films. However, when PET is formed or injection molded into thick parts, its high crystallization rate is likely to cause whitening in the parts due to crystallization in the fabrication step. To avoid this disadvantage, PET can be modified with a comonomer to reduce the crystallization rate. Examples of comonomers used to modify PET are isophthalic acid and 1,4-cyclohexanedimethanol (CHDM). These modified PET resins are commercially available.
However, these CHDM modified PET resins have Tgs less than 95° C., and therefore, their use in applications requiring a high heat resistance, for example, injection molded cups and bowls to be placed in a dishwasher; or a sign for outdoor use in hot climates, is limited.
The crystallization rate for PET can be modified by a variety of comonomers. SPG is effective at increasing the Tg when used as comonomer for PET. However, for PET modified by SPG, U.S. Pat. No. 6,447,859 indicates that the molecular weight distribution can be regulated within the range of 2.5 to 12.0 by appropriately selecting the addition amount and addition timing of SPG, the molecular weight of the polyester, the polymerization temperature, and additives. In other words, the degree of branching, as indicated by the molecular weight distribution, formed in these polyesters is controlled by a reaction conditions. As a result, it can be difficult to control the degree of branching and therefore the melt viscosity profile consistently during the production of SPG modified PET due to changes in process conditions. It is more desirable to control branching by using a branching agent that is added to a polyester and not formed in-situ. In this case, the degree of branching or molecular weight distribution would be directly proportional to the amount of branching agent added and not heavily influenced by reaction conditions as in the case when reactions take place with SPG to form branching agents.
PCT (polycyclohexylene-dimethylene-terephthalate) is also industrially valuable polyester because of its superiority in mechanical properties. For example, PCT has been widely used in various applications such as fibers and melt-blown fabrics. However, like PET when PCT is formed or injection molded into thick parts, its high crystallization rate will cause whitening in the part due to crystallization in the fabrication step. To avoid this disadvantage, like PET, PCT can be modified with a comonomer to reduce the crystallization rate. Examples of comonomers used to modify PCT are isophthalic acid and ethylene glycol. These ethylene glycol or isophthalic acid modified PCTs are known in the art and are commercially available. However, these modified PCTs have Tgs less than 95° C., so their use in applications requiring a high heat resistance, for example, injection molded cups and bowls placed in a dishwasher or outdoor signs in hot climates, is limited.
There is a need in the art for reducing the crystallization rate of PCT while increasing the Tg above 95° C.
Further, in one embodiment, there is a need in the art for polyesters having better processability, good color, molecular weight distribution of less than 2.5 or less than or equal to 2 or less during polyester synthesis in the melt (, Tgs greater than 95° C., articles made therefrom; and processes of making these polyester compositions.